Home / Backpacking Skills / Advanced Backpacking Skills / Ultralight Bivy Sack Guide

Ultralight Bivy Sack Guide

Mountain Laurel Designs Superlight Bivy Sack
Mountain Laurel Designs Superlight Bivy Sack – 6.3 ounces

Ultralight bivy sacks are used by backpackers using floorless shelters to protect their sleeping bag or quilt from moisture and their head from biting insects, as a kind of substitute for the inner tent that you find in more conventional double wall tent setups. They’re basically sleeping bag covers with a waterproof floor that have mosquito netting over the face.

Most ultralight bivy sacks have just enough room inside for a sleeping pad and a sleeping bag or sleeping quilt, but sleeping in them really isn’t as claustrophobic as it might look and there’s still plenty of room to sleep on your side or prop your head up on your arm to read at night.

Ultralight Bivy Sack under a floorless pyramid trap
Ultralight Bivy Sack under a floorless pyramid trap

Many ultralight bivy sacks have a cord loop that you can tie to the underside of your shelter to hold the bug netting up off of your face when you sleep. You can also tie this cord to the rafters in a trail shelter and use your bivy to protect you from bugs and mice. Your bivy sack also provides a wee bit more privacy inside a shelter if you want to change your clothes.

Ultralight Bivy Sack in use inside an Appalachian Trail Shelter
Ultralight Bivy Sack in use inside an Appalachian Trail Shelter

When sleeping under a tarp, a bivy sack helps protect your sleep system from splash-back, which occurs when rain bounces off the ground and under your tarp and on your sleeping gear. This can be an issue if you have a narrow rectangular tarp, but is less of a concern if you use a wide or shaped tarp, whose sides are tied close to the ground in bad weather.

Most ultralight bivy sacks don’t impart much extra warmth to your sleeping bag or quilt at night, although they will block a breeze from chilling you. They are often made with a top fabric that is quite breathable to help vent any condensation build-up at night. The top fabric is so light weight that you can sleep on top of your sleeping bag or quilt and use like a sheet in very hot weather to keep the bugs from biting your legs and arms.

Ultralight Bivy Sack under a floorless Cuben Fiber Tarp
Ultralight Bivy Sack under a floorless Cuben Fiber Tarp

Ultralight bivy sacks are quite delicate pieces of gear made by hand and must be treated gently if you want them to last. If you buy a bivy sack that has a zipper, it’s good to lubricate it periodically with McNet’s Zip Tech so it doesn’t snag, and to dry your bivy sack out after every trip you take to avoid mildew. If you see black mildew spots begin to form on the bivy fabric, washing your bivy sack out with diluted Mirazyme will eliminate it and prevent damage to the fabric.

Other Types of Bivy Sacks

Most of the bivy sacks that you’ll find at REI or other outdoor retailers are two or three times heavier than the hand-made ultralight bivy sacks that can buy from smaller gear manufacturers. While many of these are intended primarily for winter use in a snow cave or shelter, some have bug netting over the face and can also be used in warmer weather, such as the: Black Diamond Twilight Bivy or Mountain Hardware Dry.Q Bivy Sack.

Some bivy sacks are so bombproof that they can even be classified as tents because they include their own poles to pitch, including the Outdoor Research Advanced Bivy Sack and the Black Diamond Bipod Bivy Bag.

Unlike Ultralight Bivy sacks, none of these other bivy types are intended to be used with floorless shelters, but are meant to be standalone shelters in their own right. It is confusing that they’re all called bivy sacks, so I hope this post has helped clarify the differences between them.

Most Popular Searches

  • ultralight bivy
  • ultralight bivy sack
  • lightest bivy sack


  1. A couple of things: first, why do you use a ground cloth with the bivysack as shown in picture number two? I thougt that using a bivysack also would eliminate the need for another layer between the sleepingmat and the ground? If not, then what is the purpose of the bottom of the bivysack? To save weight, or at least to minimize what you need to bring along, a groundcloth in addition to a bivysack seems to me as overkill. If the case is som that the bottom of the bivysack is to frail, then why not use the same material is in the groundcloth for the bottom of the bivysack?

    Secondly: I believe the term bivysack came first, and “Ultralight” second. So the ultralight industry should rather rename their product to clarify the difference.

    • I also do that, using a very light polycro ground sheet. For me, the main reason is that I have some extra space for everything else that is also protected from the wet ground and it keeps the bivy bag cleaner (bivy bag with sleep gear inside goes into the backpack in the morning, the small ground sheet stays in the mesh pockets outside). It’s worth the extra ounce for me.

  2. I would add the bivys from http://www.titaniumgoat.com. I use their Ptarmigan bag and am very happy. Used it on my coast-to-coast through Scotland under a GG Twin Tarp and it worked well. No affiliation, just saying.

  3. An inexpensive, albeit non-lightweight, alternative is a US Military Gore-Tex bivy. These can be purchased for about $50. I’ve been using mine for my monthly “microadventure” in which the purpose is to spend a night outside under the stars without a tent. So far, this has worked great even in cold, snowy conditions when pared with a $10 tarp from the hardware store.

    • I have a love-hate relationship with the Army bivy sack. Love it because it has kept me dry through one field exercise after another since 2005. I mean the thing is made of triple-layer goretex – absolutely waterproof even if you sleep the night in a puddle (I have). It’s also surprisingly breathable – i recall waking up one morning, toasty warm and dry, emerging from the bag to discover frost covering everything around me.
      Hate? Hate the weight and bulk. 2.5 lbs. But the army is not the place for wimpy gear that needs to be babied. This bivy sack is the last fortress of a soldier’s morale, a refuge against the elements that simply will not fail.

  4. Philip- can the ultralight, delicate bivy sacks be treated with Permethrin or other treatments to enhance their anti-insect effectiveness if using just as a light sheet in summer time? Or do they simply not need this treatment?

  5. i really enjoy using my ultralight bivy. no effort to set up camp in nice weather.

  6. I have a number of Bivy’s due to them appearing on the market over the years. One from Eureka, MSR, Outdoor Research and the Military ECWS Bivy. I enjoy them in Fall and early Spring but after that heat and humidity and humidity build up in them relegates them to the Storage Locker especially now that I am in the humid South..They were great in the High and Dry Eastern Sierra. . I soon learned as seen in the Photos of bringing a Lightweight tarp along, I use a Sil-nylon 10×10 which is perfect should the weather turn nasty..But if you guaranteed great weather for your weekend hike I’d leave the Tarp at home….I prefer the Outdoor Research Bivy over the others because it has a Nylon Hoop design which keeps the material off your face and allows for some Venting of the moist air from your Breath which can really dampen your bag over night and it packs ups so small to…

  7. Not required but polycro (plastic) generally has a higher hydrostatic head than silnylon.

  8. I also use a thin sheet of polycro, or even tyvek, as much of my camping in the spring and summer is in rocky deserts or prickly pine areas.

  9. I’ve read your reviews of the MLD ultralight and MLD bug bivies and I wonder your recommendation of which would be better to use in a trail shelter in the summer.

  10. I have the exact same setup with an MLD Gracetarp and their Ultralight .Bivy. I love it.

Leave a Reply

Your email address will not be published. Required fields are marked *